shell: Let panels have their own commandline flags

Add a class method to CcPanel to get a GOptionGroup which will be added
to the main commandline parser. This gives panels the chance to have
commandline "--flags" in addition to the already available parameters.

This changes changes the way parameters are passed to panels: the first
entry in the GVariant array is always the a{sv} dictionary of
commandline flags, followed by the remaining free-form arguments.

https://bugzilla.gnome.org/show_bug.cgi?id=696054

shell/cc-panel.c

@@ -259,3 +259,15 @@ cc_panel_get_help_uri (CcPanel *panel)
 
   return NULL;
 }
+
+GOptionGroup *
+cc_panel_get_option_group (GType            panel_type,
+                           GVariantBuilder *builder)
+{
+  GOptionGroup *options = NULL;
+  CcPanelClass *class = CC_PANEL_CLASS (g_type_class_ref (panel_type));
+  if (class->get_option_group != NULL)
+    options = class->get_option_group (builder);
+  g_type_class_unref (class);
+  return options;
+}